Method and apparatus for investigating earth formations traversed by boreholes



METHOD AND APPARATUS FOR INVESTIGATING EARTH FORMATIONS TRAVERSED BY A BOREHOLE f Filed June 15, 1942 4 Sheets-Sheet 1 Dec. 30, 1947. HENRi-GEoRGEs DOLL. 2,433,746

. 2,433,746 METHOD AND APPARATUS Fon INVESTIGATING EARTH FQRMATIONS TRAVERSED BY A BOREHOLE Filed June 13, 1942 f4 Sheets-Sheet 2 Dec. 30, 1947- HENRI-GEORGES DOLL REcoRDERs soz/Rc: 0F ELECTRICAL ENERGY RECORDER m 4 Q @Cr INVENroR "d '55 x3 Henri-Ganges' Dall 2 BY w a.' y i METHOD AND APPARATUS FOR INVESTIGATING EARTH FORMATIONS TRAVERSED BY A BOREHOLE Filed June 15, 1942 'A 4 Sheets-Sheet 3 Dil-.30,1941 v l-l'ENRx-GE'ORGES DOLL "2,433,746

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l lienri-eorges Dall HENRI-GEORGES DOLL Dec. 30, 1947.

' METHOD AND APPARATUS FOR INVESTIGATING EARTH FORMATIONS TRAVERSED BY A BOREHOLE Filed June 13, 1942 4 Sheets-Sheet 4 ATTONEYS INVENTOR Henri-@forges D011 Psfented Dee. 3o, 1947 METHOD AND APPARATUS 'FOR INVESTI- l GATING EARTH FORMATIONS TRAV- ERSED BY BOREHOLES Henri-Georges Doll, Houston, Tex., asslgnor to Schlumberger Well Surveying Corporation, v`ljlouston, Tex., a corporation of Delaware Application June 13, 1942, Serial No. 446,888 z claims (ci. 11s- 182i The present invention relates to methods and apparatus for investigating earth formations traversed by a bore hole, and more particularly to a new and improved method and apparatus for determining the depth and thickness of permeable formations traversed by a bore hole.

In the present practice, the location and vertical extent of permeable formations traversed by a bore hole are usually determined by weil logglngf methods of the type disclosed in prior Patent No. 1,913,293 to Conrad Schlumberger. Ac-l cordlng to the method described in that patent,

indications are obtained of naturally occurring potentials between an electrode moved through the bore hole and a suitably chosen point of ref-` erence. These naturally occurring potentials are continuous potentials of variable vmagnitude which are known to be indicative of the per meability of the surrounding formations.

Naturally-occurring potentials ina bore hole containing a column of liquid generally include potentials produced by electrochemical effects and 'potentials produced by the phenomenon known as electroflltration. Most bore holes encountered in practice contain a column of liquid or drilling mud which is an electrolyte. The reactions between this electrolyte, the materials comprising the surrounding formations, and other electrolytes contained in such formations partly account for the electrochemical potentials that exist in the bore hole.

'I'he infiltration of the bore hole liquid into porous or permeable formations in which the fluid pressure is lower than the "bore hole liquid pressure gives rise to a flow of electric current' in the same direction as the flow of liquid. This current ow produces potentials in passing through the electrolyte or electrolytes present in the vicinity of the permeable formations.

yThese potentials constitute another component of naturally-occurring potentials in a bore hole.

In addition to the two component potentials referred to above, there may sometimes be other continuous potentials present which are not related to the permeability of the formations. spurious potentials of this nature are generally found where there are relatively large telluric currents flowing through the earth in the vicinity of the well which is being logged. In 'such cases great care must be exercised in practicing prior art methodsin order to obtain accurate results.

Patent No, 1,913,293 points out the conditions under which potentials are encountered in the borehole in the method therein described. It also points out that under the established operating 65 tion.

conditions encountered in the field and now well .known in the art the electroiiltration potential is proportional to the pressure causing such filtration. f

It is an object of the invention, accordingly, to provide for use under the conditions in the leld disclosed in said Patent No. 1,913,293 a novel method and apparatus for obtaining indications in abore hole, of naturally occurring potentials that are related to the permeability of the formations. apart from other spurious potentials that may be present. These potentials-are now referred to in the art as electroltration potentials or streaming potentials'and have been widely employed for more than ten years to locate permeable strata in situ traversed by a drill hole.

Another object of the invention is to provide a novel method and apparatus for modifying the characteristics of electroltration potentials as they have been found to exist in bore holes whereby they may be readily distinguished from other continuous potentials that may exist in the bore hole.

A further object of the invention is to provide a novel method and apparatus of the above character by virtue of which indications may be obtained of electrofiltration potentials existing in a bore hole containing a column of liquid.

Another object of the 'invention is to provide a novel method and apparatus of the above character for transmuting continuous electroflltration potentials in a bore hole into periodically variperiodic variations in the pressure of the liquid contained in a bore hole for the purpose of pro- `ducing periodically variable electroiiltration potentials therein.

Still another object of the invention is to provide a lnovel method and apparatus for simultaneously obtaining indications of periodically variable electrofiltration potentials and at least one other electrical valuerelated to a different subject of interest ina bore hole.

The invention is based, in part, on the fact that the electroltration potentials existing in a bore hole at the level of a permeable formation are related to the permeability of the formation and are ya 4function of the difference in pressure between the bore hole liquid at the level of the formation and the fiuid contained in the forma.-

` -to the pressure differential. This relation may b expressed as gPD 417th where 'g is the potential of the double layer at the solid-fluid contact, P is the pressure differential, D is the dielectric constant, n is the viscosity, and lc the specific conductivity.

Experiments have shown that the streaming potentials are a linear function of the pressure (page 459, Experimental and Theoretical Electrochemistry, by Dole, McGraw-Hill, 1935).

In terms of oil eld practice, the filter cake formed on the wail of the bore hole opposite a permeable formation together with that formation itself constitute the porous material or system of capillaries, and the bore hole fluid is the liquid. It follows then that if the pressure P is periodically. varied, a corresponding variation will be noted in the potential E. In accordance with this principle and the present invention, if the pressure is varied rapidly enough to produce a fluctuating streaming potential, this variable potential may be considered as an alternating potential. Then for practical purposes the provoked streaming potentials may appear as A. C. and the previously existing natural potentials as D. C., upon which basis they may be separated. A condenser in series in the measuring circuit would block the natural potentials and pass the provoked streaming potentials. Preferably a lter is included or substituted to block A. C. currents of frequencies other than that of the vibration frequency.

According to the invention, a periodic and localized variation in the pressure of the bore hole liquid is produced at diiferent depths in the bore hole. Whenever the variation in pressure occurs in the vicinity of a relatively permeable formation in which the fluid pressure is different from the bore hole liquid pressure, a corresponding periodic variation in the potential caused by I electroltr'ation is observed. The component causedby electrochemical action, however, is apparently not changed by the modification of the pressure of the bore hole liquid. The periodically varying electroltration potential, therefore, may be readily measured in any suitable manner, apart from any other continuous potentials that may be present.

` The invention thus enables accurate indications of electroiltration potentials to be obtained in a bore hole with ease, even where there are relatively strong'telluric currents present. Furthermore, the periodically variable electroflltration potentials may be transmitted from dierent locations in the bore hole to measuring apparatus at the surface of the earth through a common conductor, simultaneously with direct current, or dierent frequency alternating' current, indications of another object of interest in a bore hole. be, for example, the electrical resistivity of the formations at different depths, obtained in ac- The other subject of interest may cordance with the method disclosed in prior Patent No. 1,819,923 or any other electrical characteristic of the formations.

The invention may be better understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

Fig. `l is a schematic diagram in longitudinal section of apparatus constructed according. to the invention for producing periodically variable potentials related to electroflltration potentials in a bore hole;

Fig. 2 illustrates a curve of spontaneous potentials such as might be'obtained by means of the apparatus shown in Fig. 1;

Fig. 3 is a schematic diagram of a modified form of apparatus constructed according to the invention; f

Fig. 4 illustrates schematically a further embodiment of the invention;

Fig. 5 shows a curve of spontaneous potentials such` as might be obtained by means of the apparatus shown in Fig. 4;

Fig. 6 is a schematic diagram in longitudinal section of a further modification of the invention utilizing a liquid operated turbine in the bore hole for producing pressure variations therein;

Fig. 6a is a view in section taken along linela-Ga of Fig. 6 and looking in the direction of the arrows;

Figs. 6b and 6c are sectional views" of a portion of the apparatus shown in Fig. 6, taken along lines 6b6b and lic-6c, respectively, and looking in the direction of the arrows;

Fig. 7 shows the apparatus of Fig. 6 in position in a bore hole and illustrates schematically the electrical connections employed;

Fig. 8 shows an illustrative curve which might be obtained with the apparatus of Figs. 6 and 7; and

Fig. 9 is a schematic diagram of apparatus constructed according to the invention for simultaneously obtaining indications of alternating electroflltration potentials and of another subject of interest in a bore hole.

Referring now to Fig. 1, the invention is shown embodied in apparatus I0 for producing periodic variations in the pressure of the liquid II at the level of a relatively permeable formation I3 traversed by a bore hole I2. It will be assumed in\ crank arm I1 on which is mounted a weight I8.

The crank arm I1 is mounted in suitable bearings I9 and 20 for rotation about a vertical axis.

The crank arm I'l drives, through conventional gearing 2i, a second crank arm 22, journalled in suitable bearings 23 and 24 for vertical rotation within the casing I4 and having a Weight 25 mounted thereon. It will be noted that the gearing 2| causes the crank arms I1 and 22 to rotate in opposite directions, when the motor I5 is in operation.

One terminal of the motor I5 is connected to a conductor 26 in the supporting cable 21, the upper end of which is connected in series with a conventional type switch 28 and a suitable source l ofelectrical energy 29, the other terminal of its other terminal is grounded to the body of the apparatus I4 at the point 3I. If any interference is encountered between the current and the recording circuits, it may be desirable to use a second conductor in the cable 21 instead of the ground return.

In operation, the switch 28 at the surface of the earth is closed andthe apparatus I0 is moved through the bore hole I2. This energizes the motor i5, causing it to drive the crank arms I1 and 22 in opposite directions. The result of this rotation of the weights I8 andV 25, respectively, on the crank arms I1 and 22, respectively, is to impart 'an extremely vigorous vibrating motion to the apparatus i0 in a plane perpendicular to the paper. This vigorous vibration produces periodic increases and decreases in thepressure of the bore hole liquid at opposite sides of the apparatus i0. example, create such periodic increases and decreases in pressure at -a frequency of 600 cycles per second. Measurable potentials have been obtained in the laboratory with frequencies as high as 30,000 per second. These frequencies are referred to merely by way of example because the principle of operation is not limited by the frequencies employed, so far as is known and believed. Limitations of operativeness of the ap- The apparatus described may. for I paratus employed in connection with the pracp tice of the method may, of course, impose practical limitations, as is to be expected in the mechanical and electrical arts.

As described above, the periodic variation in the pressure of the bore hole liquid II as the `apparatus passes the formation I`3 produces a corresponding periodic variation in the electroltration potentials existing in the bore hole I2'v in the vicinity of the porous formation I3. These electroltration potentials may be measured in any desired manner by any suitable apparatus. Such apparatus may comprise, for example, a relatively small, insulated electrode 32 of any desired shape, mounted on the outside of the casing I4, preferably on a radius perpendicular to a line passing through the centers of the bearings I9 and 23. The electrode 32 may be connected through a conductor 33 in the supporting cable 21 to one terminal of a conventional alternating current potential indicating instrument 84, preferably of the recording type, the other terminal of `which may be connected to a ground point 35 atthe surface of the earth.

A representative curve such asmight be obtained by the recording apparatus 34 is shown in Fig. 2, from which it appears that the spontaneous potential curve is modulatedin the vicinity of the porous formation I3 at the frequency of vibration of the apparatus Ill in the bore hole.

of the casing I4 constitute a closed chamber 38' which is preferably filled with a substantially incompressible liquid such as oil, for example,

which serves to equalize the relatively great pressure of the bore hole liquid Ii contained in the bore hole I2. i

The variation in pressure ofthe bore hole liquid I I is produced by periodically reciprocating the "Sylphon bellows 31 by means of a shaft 39'. The lower end of the shaft 39 is secured to the lower closure member 40 of the Sylphon" bellows 31 and its upper end is connected to the piston 4I oi a conventional type dashpot 42 which is slidably mounted in a bore 43 formed in a support 44 mounted in the casing I4. The upper closure member 45 of the dashpot 42 engages a cam memberl 46 rotatably mounted within the casing I4 and provided with a gear 41 `which is driven by a corresponding gear 48 mounted on the shaft of the motor I5. The purpose of the dashpot 42 is to permit the Sylphon" bellows 31 to assume a position for which the pressure in the chamber 36 will be equalto the pressure of the bore hole liquid I I.

When the switch 28 at the surface of the earth is closed, the motor I5 is energized, causing the cam 46 to rotate and periodically reciprocate the dashpot 42. The action of the dashpot 42 is such that it does not respond to the rapid reciprocating motion produced by the cam 46. Accordingly,

the'reci'procatory motion produced by the cam 46 "is directly transmitted to the shaft 39,- so that the Sylphon bellows 31 is reciprocated at a rate determined by the speed of rotationof the motor I5.

Since the "Sylphon bellows 31 is in communication with the bore hole liquid II through suitable apertures 36 formed in the lower extremvity of the casing I4, its reciprocatory motion produces periodic variations in the pressure of the bore hole liquid I'I in the vicinity of the permeable formation I3. The periodically varying electroflltration potentials thus produced may be measured in essentially the same manner as described above in connection with Fig. 1. Or,

if desired, the recorder( 34 may be connected through a third conductor 49 in the supporting cable 21 to an electrode 50 mounted a relatively short distance above the electrode 99.

Periodic variations in the pressure` of the bore hole liquid Il may also b e produced by directing jets of liquid periodically in opposite directions toward the Wall of the bore hole I2. Apparatus ,suitable for this purpose is shown in Fig. 4 of the At the junction of the conduits 55 and 56 is" mounted a conventional type clapper valve 51 which is adapted to be moved periodically from the position shown in full lines to the positionv shown in dotted lines' in the drawing. This may be accomplished in any suitable manner, as, for

example, by mounting an arm 58 having a longitudinal slot 59 therein on the shaft of the clapper valve v51. Within the slot 59 is adapted to be received a pin 30 secured ona rotatable disc 6I A driven by a conventional type electric motor 62.

One terminal of the motor 621s connected to the conductor 23 in the supporting cable and its other terminal is grounded to the casing 5 I at the point 63.

In operation, the switch 26 is closed at the surface oi the earth, supplying electrical energy from the source 29 to the electric motor 62. The disc 6I begins to rotate and moves the clapper valve 51 periodically from the position shown in heavy lines to that shown in dotted lines. Meanwhile, the casing I is moved upwardly through the bore hole by applying tension to the supporting cable 21 so that the bore hole fluid II is directed'through the cpnduit 53 and alternately through the conduits 55 and 66.

The letting action of the bore hole fluid I I produced in its passage through the diametrically opposite conduits 55 and 56 produces a vigorous lateral vibration of the casing 5I and alternately compresses the bore hole liquid II in the vicinity of the outlets of the conduits 55 and 66. As described above, the periodic variation in the pressure of the bore hole liquid II is accompanied by corresponding variations in the electroflltration potentials existing in the vicinity oi permeable formations.

These potential variations may be picked up as described above or by measuring the potential diierenceY between two insulated electrodes 64 and 65 located on opposite sides of the casing 5I in the vicinity of the conduits 55 and 56. The electrodes 64 and 65 may be connected to the conductors 33 and 49 in the supporting cable and the potential differences picked up are measured by the recording apparatus 34 at the surface of the earth.

A representative curve, such as might be obtained with the apparatus shown in Fig. 4, is illustrated in Fig. 5 of the drawings. As shown in this figure, the curve of spontaneous potentials at the level of relatively permeable formations has approximately a sinusoidal shape and its frequency is the same as the frequency of vibration of the apparatus in the bore hole.

Comparison of Figures 2 and 5 shows that at least two types of curves may be produced by employing this invention: flrst,a curve which represents values related to electrochemical potentials as well as electrofltration potentials, as shown in Figure 2. and second, `a curve which indicates values corresponding only to electroflltration potentials, as'indicated 'in Figure 5.

Referring again to Figure 4, it will be noted that the recorder 34 indicates the difference in potential between electrodes 64 and 65 which are positioned at the same depth in the bore hole but on opposite sides of the sonde. The electrochemical potential and other natural potentials appearing on eachof the electrodes 64 and 65 are therefore the same, so that they do not appear in a measuring circuit; the only potentials measured by recorder 34 are the variations in streaming potentials between electrodes 64 and 65 produced by the vibration of the bore hole equipment.

In the embodiment illustrated in Fig. 6 of the drawings, a hydraulic turbine is employed to produce periodic variations in the pressure of the bore hole liquid at different depths in the bore hole. Referring to Fig. 6, the apparatus comy prises a casing 66 at the upper extremity of which are mounted a plurality of flexible ns 61 which serve to maintain the casing 66 in fixed relation to the axis of the bore hole I2. Although any opening and the turbine outlet 8|.

Secured to the lower portions of the flexible i-lns 61 are a plurality of flaps 68 made of any suitable material, such as canvas, for example, which extend between the outer edges of adjacent flns 61, as shown, to form a funnelfor directing the bore hole liquid II to a plurality of forming a large funnel as shown in the drawings.

The fluid inlets 69 in the casing 66 communicate with a chamber 10, which in turn communicates with the inlet port 1I of a conventional type hydraulic turbine 12. The turbine 12 comprises two sets of stator blades 13 and 14 between which is rotatably mounted a rotor 16 having a plurality of blades 16 mounted thereon.

In order to provide for periodic pressure variations in the bore hole liquid I I, a bottom closure member 11 is provided between two adjacent blades 16 on the turbine rotor 15, as shown in greater detail in Fig. 6j. Also, similar bottom closure members 16 and 18a are provided between two pairs of adjacent xed blades 14 below the rotor 16 and outlets 80 and 80a are formed directly opposite the two pairs of adjacent blades 14 which have been so -closed off. While the device will function with only one outlet 80, it is preferable to provide two in order to increase the vibrational energy imparted to the bore hole liquid. It is also desirable to have one or more less blades on the rotor 15 than there are stator blades 14. With this construction, the closure member 11 on the rotor 16 subtends a greater arc than the passages between the blades 14 to which the bottom closure members 18 and 18a are secured, and reduces to a minimum any leakage from the outlets and 80a when the rotor bottom closure member 11 lies over the stator blades 14 to which the closure members 18 and 18a are secured.

In operation, the casing 66 is lowered to the bottom of the'bore hole on the supporting cable 21. While-it is being lowered, the aps 68 between the fins 61 are collapsed so that they do not impede the movement of the apparatus in the bore hole. The apparatus is then raised, whereupon the aps 68 expand to form a funnel, as shown in the figure. This funneldirects the bore hole ,liquid II through the fluid inlets 69 and the chamber 10 to the turbine inlet 1I, causing the rotor 15 thereof to rotate at a speed which depends upon the speed at which the apparatus is moved through the bore hole I2.

So long as the bottom closure member 11 between the two rotor blades 16 is not directly over either of the two pairs of adjacent blades 14 to which the bottom closure members 18 and 18a are secured, water will pass through the turbine 12 and through the outlets 80 and 60a from which it will be jetted radially against the wall of the bore hole I2. The water will also be continuously exhausted through the turbine outlet 8| However, when the bottom closure 11 moves directly over either of the two pairs of adjacent Iblades 14 to which the bottom closures 18 and 16a are secured, practically no iluid will pass through the corresponding opening 80 or 80a but subf stantially all the uid Will pass through the other as the rotor 16 rotates, intermittent and alter- Accordingly,

nate jets of water will be directed from the outlets 80 and 80a, thereby producing a lateral vibration of the casing. 66 and periodically varying the pressure of the bore hole liquid I I at the level of the apparatus in the bore hole.

The variations in the electroilltration potential produced by the periodic variations in the pressure of the bore hole liquidy Ii may be picked up by a pair of insulated electrodes 82 and 83 located on opposite sides ofthe casing 66 near the outlets 80a and 8|),v respectively. The electrodes 82 and 83 may be connected to the conductors 33 and 49 in the supporting cable 21 (Fig. 7) to a conventional type rectifier 84 at the surface of vthe earth, the direct current outputhof which is impressed upon the recording apparatus 34. The rectifier 84 serves to rectify the periodically varying eiectroltration potential which is produced in the bore hole i2, so that the curve recorded has substantially the shape shown in Fig, 8 of the drawings,

Where a single outlet 881s employed, the presmeans, there are no current carrying-conductors in the supporting cable 21 other than the conductors 33 and 49, which transmit'the electronitration potentials to the surfaceof the earth.

. Accordingly, the electroilltration potentials picked up may be very accurately measured.

The apparatus illustrated in Fig. 9 of the drawings shows how alternatingy electroltration `po- Variations in the continuous potential existing at the electrode 81 are transmitted through the conductor 33 in the supporting cable 21 to one terminal of a conventional type lfilter 92 which passes direct current but discriminates against alternating current, the other terminal of which is connected to ground at the pointf35 at the surface of the earth. The output of the filter 92 is fed to a conventional type, continuous current recording instrument 94 which provides a curve that is a function of the electrical resistivity' of the formations at different depths in the bore hole.

The alternating electroflltration potentials produced by variations in the pressure ofthe y bore hole liquid ii created by the apparatus 85 are also picked up by the electrode 81. These potentials are transmitted through the conductor 33'to one terminal of a second filter 95 which passes alternating current of the frequency of the alternating electrofiltration potentials but discriminates against continuous current, the other terminal of which is connected to the ground at the point 35. The' output of the filter indications might be obtained simultaneously of alternating electrolltration potentials and of any other subject of interest in the bore hole, such tentials may be lmeasured simultaneously with potentials that are a function of another sub.

ject of interest in the bore hole. While any one of a number of dierent subjects of ,interest may as, for example, the electrical impedance of the formations or of the temperature existing at dierent depths in the bore hole. In such cases, itis only necessary that the electrical signal which is a function of the different subject of interest in the bore hole be either a direct current signal or an alternating current signal of different frequency from the frequency of the alternating electrofiltration potentials.

In each of the embodiments described above,

diiferent measuring circuits are employed for obthe electrical resistivity of earth formations traversed by a drill hole. Moreover,.although either direct current, pulsated direct current, or an alternating current offdifferent frequency from the alternating electroiltration potentials may be employed in such measurements. it will be assumed -in the following description that direct current is used. Preferably, the value of the direct current employed should be high enough to renderthe influence of natural potentials wholly insignificant. Y

Considering now Fig, 9, the investigating apparatus 85 in the bore hole may comprise any pf the modications described above, such as, for example, the embodiment shown in Fig. 3 of the drawings. Mounted on the apparatus 85 are two spaced apart, insulated electrodes 86 and 81. The electrode 86 is connected through the conductor 26 in the supporting cable 21 to a direct current source of electrical energy 29 at the surface of the earth, the other terminal of which is connected to ground at the point 30. This 'circuit creates in the intervening earth formations an electric field which impresses upon the electrode 81 a potential that is la function of the electrical -resistivity of the formations, as described in the prior United States Patent No. 1,819,923.

'taining indications of electroilltration potentials in a bore hole. It is to be understood that the different measuring circuits shown may be used indifierently with the different embodiments shown, depending on the type of curve desired. For example, in Figure 1, indications of electroltration potentials might be obtained by making measurements between the electrode 32 and another electrode located at the same levelon the other side of the casing I4 opposite the electrode 32. A measuring circuit of this type is shown in Figure 4 of the drawings. As a further modication, indications might be obtained of the voltages between each of two such electrodes and ground.

The invention thus provides a new and improved method and apparatus for obtaining indications of electrofiltration potentials apart `from other continuous potentials that may be present in a bore hole. By producing a localized vand periodically variable pressure variation at different depths in the bore hole, continuous electroflltration potentials may be changed to alternating potentials that can be readily sep; arated from other continuous potentials which may be encountered. Accordingly, the invention enables accurate indications of electroltration 11 special precautions even in wells located in the vicinity of relatively strong tlluric currents.

The several embodiments described above are intended to be illustrative and not restrictive. Numerous modifications in form and detail may be made in those embodiments within the scope of the following claims.

I claim:

1. A method of locatingr permeable earth formations traversed by a bore hole containing a column of liquid comprising lowering a body through the bore hole and producing at said body artificially periodically fluctuating pressure at a relatively rapid r-ate above that to which a direct current meter is appreclably responsive in the said liquid at the level of a relatively permeable formation, thereby creating periodically variable flow of the liquid into the permeable formation to generate variable alternating electroltration potentials in the vicinity of said permeable formation, simultaneously lowering an electrode with the said body and in such relation thereto as to be substantially at the same level as the body, placing a second electrode at a position where it is not subject to the same instantaneous, fluctuating, liquid pressure as said first electrode, transmitting to the surface alternating potentials picked up by said electrodes and produced by said generated alternating electroltration potentials, and obtaining indications of said alternating potentials.

2. Apparatus for locating permeable earth formations traversed by a bore hole containing a column of liquid comprising a body adapted to be lowered through the bore hole and having means for producing artificially periodically fluctuating pressure at a relatively rapid rate above that to which a direct current meter is appreciably responsive in the said liquid Aat the level of a relatively permeable formation, thereby creating periodically variable flow of the liquid into the permeable formation to generate variable alternating electroltration potentials in the vicinity of said permeable formation, a ilrst electrode assembled to said body so as to be movable at substantially the same level therewith, a. second electrode located at a position where it is not subject to the same instantaneous uctuating liquid pressure as said ilrst electrode, circuit means transmitting to the surface alternating potentials picked up by said electrodes and produced by said generated alternating electroflltration potentials, and means for providing indications of 'said alternating potentials.

HENRI-GEORGES DOLL.

REFERENCES CITED The following references arev of record in the le of this patent:

UNITED STATES PATENTS Number Name Date v2,190,686 Slichter Feb. 20, 1940 2,304,051 Beers Dec. 1, 1942 988,537 Aufiero Apr. 4, 1911 2,248,073 Gage July 8, 1941 1,475,385 Hecht Nov. 27, 1923 2,276,613 Gordon Mar. 17, 1942 1,955,815 Lauterback Apr. 24, 1934 1,675,560 Jubien July 3, 1928 2,206,863 Cloud July 9, 1940 2,085,664 Karcher June 29, 1937 2,104,440 Statham Jan. 4, 1938 2,156,259 Blau May 2, 1939 2,181,601 Jakosky Nov. 28, 1939 1,826,961 Slichter Oct. 13, 1931 1,913,293 Schlumberger June 6, 1933 2,132,807 Rust Oct. 11, 1938 

